function Parent(){
    this.a = 1
    this.b = [1,2,this.a]
    this.c = {demo: 5}
    this.show = function(){
        console.log(this.a,this.b,this.c.demo)
    }
}

function Child(){
    this.a = 2
    this.change = function(){
        this.b.push(this.a)
        this.a = this.b.length
        this.c.demo = this.a++
    }
}

Child.prototype = new Parent()
let parent = new Parent()
let child1 = new Child()
let child2 = new Child()
child1.a = 11
child2.a = 12
parent.show()
child1.show()
child2.show()
child1.change()
child2.change()
parent.show()
child1.show()
child2.show()

